Wednesday, July 18, 2012

850. Origins of Modern Humans: A Conversation with Chris Stringer

Paleoanthropologist Chris Stringer 

By John Noble Wilford, The New York Times, July 17, 2012

Who are we, and where did we come from? Scientists studying the origin of modern humans, Homo sapiens, keep reaching deeper in time to answer those questions — toward the last common ancestor of great apes and humans, then forward to the emergence of people more and more like us in body and behavior.

Their research is advancing on three fronts. Fossils of skulls and bones expose anatomical changes. Genetics reveals the timing and place of the Eve of modern humans.

And archaeology turns up ancient artifacts reflecting abstract and creative thought, and a growing self-awareness. Just last month, researchers made the startling announcement that Stone Age paintings in Spanish caves were much older than previously thought, from a time when Neanderthals were still alive.

To help make sense of this cascade of new information, a leading authority on modern human evolution — the British paleoanthropologist Chris Stringer — recently sat for an interview in New York that ranged across many recent developments: the evidence of interbreeding between Neanderthals and Homo sapiens; the puzzling extinct species of little people nicknamed the hobbits; and the implications of a girl’s 40,000-year-old pinkie finger found in a Siberian cave.

Dr. Stringer, an animated man of 64, is an anthropologist at the Natural History Museum in London and a fellow of the Royal Society. But he belies the image of a don: He showed up for our interview wearing a T-shirt and jeans, looking as if he had just come in from the field.

A condensed and edited version of our conversation follows. In it and in a new book, he describes a new wrinkle to the hypothesis of a recent African origin of modern Homo sapiens. His ideas may light up more debate in a contentious science.

First of all, would you explain the title of your new book?

Yes, the title is “Lone Survivors: How We Came to Be the Only Humans on Earth.” And this comes from the fact that if we went back 100,000 years, which is very recent, geologically speaking, there might have been as many as six different kinds of humans on the earth. All those other kinds have disappeared, and left us as the sole survivors.

You wrote that in 1970, when you started doing research in this field, the origin of modern humans was hardly recognized as a topic worthy of study in science. What has changed since then?

It’s been a fantastic time to be involved in the field, and even when I was writing this book in the last two years, I had to regularly go back and rewrite things I thought I’d finished with, because new developments were coming up all the time. In 1970, for some people, there was no single origin of modern humans: We evolved globally, all over the world. There was a view that in the different regions an earlier species, Homo erectus, evolved relatively seamlessly to modern humans. This idea was known as multiregionalism.

The argument went that we remained one species throughout that evolutionary process, because there was interbreeding among the different populations. It meant that the Neanderthals in Europe, for example, would be the ancestors of modern Europeans; Homo erectus in China would be the ancestor of modern Asians. And Java Man would be a distant ancestor of modern Australian aboriginal populations.

What we have seen since then is a growth in the fossil record, in our ability to date that record and to CT-scan fossils and get minute details out of them. DNA studies have had a huge impact on our field. We now have the genomes of Neanderthals and of these strange people in Siberia called the Denisovans.

Speaking of DNA, what about the African Eve? This established an approximate date for the genetic origin of modern humans, in Africa. As a leading advocate of the recent African origin, in contrast to the multiregional model, did you believe this settled the debate?

To be honest, it’s not been totally resolved, but the Mitochondrial Eve publication of 1987 was a key moment. Up to then, a few of us were arguing for a recent African origin from the fossil and archaeological evidence. But the evidence was pretty skimpy, and the majority opinion was against our view.

When this new genetic technique appeared, it seemed to give clarity to the picture. Here was an independent bit of data, from our mitochondrial DNA, inherited through females, suggesting we originated, all of us, all over the world, from a single ancestral population that lived in Africa maybe 200,000 years ago.
I came to this conclusion gradually, starting with the Neanderthals. They were the best-known ancient humans, and there was a view that they were our ancestors. I tested that model in my Ph.D. research, and I concluded the Neanderthals did not make good ancestors of modern humans, even in Europe, where we had the best data. So gradually my search moved from one region to another, to see where the evidence best fitted the idea of our origins.

It turned out that Africa was the place that had the oldest fossils of modern humans. Africa, for me, was the only place that showed a transition from archaic to modern humans.

In your book you propose that there was not one place in Africa where modern humans originated.

Earlier, influenced by the mitochondrial DNA data, I felt there was one place in Africa, a sort of garden of Eden, where we evolved, where we changed behaviorally and physically to become modern humans.
But the story is much more complicated. Even the DNA data show that essentially each of our genes has a separate evolutionary history. And so, when you look at the total picture, including the fossil data and archaeological data, there is no single spot in Africa that seems to be the place for our origins genetically.
The story is dominated by East Africa, because that’s the area that has the best preservation of the fossil record. You could say southern Africa is giving scientists the best record of behavioral evolution. They are finding evidence pretty early of processing marine resources, the use of red ocher for symbolic purposes, self-adornment with shell beads.

In my view, different parts of Africa were important at different times, to distinct human species, and this was being controlled by the climate. Africa is a huge place influenced by many different factors: the Mediterranean, the North Atlantic, the South Atlantic, the Southern Ocean, the monsoons coming off the Indian Ocean. At different times this would have produced good areas for humans and bad areas.

Populations in different areas would have flourished briefly, developed new ideas, and then maybe those populations could have died out, even — but not before exchanging genes, tools and behavioral strategies. This kept happening until we get to within the last 100,000 years, and then finally we start to see the modern pattern behaviorally and physically coalescing from these different regions to become what we call modern humans by about 60,000 years ago.

Previously, the splendid cave art of Europe influenced the view that modern behavior began there some 40,000 years ago. How firm is the new interpretation that Homo sapiens developed modern behavior as well as modern anatomy in Africa?

There were remarkable things happening in Europe at least 40,000 years ago, with the painted caves, with flutes, with the statuettes and so on. But the seeds of that revolution were sown in Africa more than 100,000 years ago. I would argue that when modern humans came out of Africa, say 60,000 years ago, fundamentally they were behaviorally modern. They took that into Europe. They took that into Asia and into Australia. So there was no single revolutionary event in Europe; this was something that was in modern humans when they came out of Africa, and the ones who stayed behind as well.

How does the discovery in Indonesia, on the island of Flores, fit in with current thinking about human migrations and lineage? Are the so-called hobbits really members of our genus Homo?

The hobbit, Homo floresiensis, is a really challenging find for everyone. There’s still a minority of scientists who don’t accept that it is a distinct human species; it’s some kind of a weird, maybe diseased form of modern human. But I think it is a genuine distinct form, and actually a very primitive form.

It’s either derived from a very primitive form of Homo erectus, maybe similar to the ones at Dmanisi in the Republic of Georgia, or it’s evidence of an earlier Africa exit, maybe before two million years ago, by something that’s pre-erectus that somehow got all the way over to the Far East and survived there in isolation, evolving for more than a million years. It’s an extraordinary story, if that’s true. And again, further evidence of how little we know about much of Asia in terms of this story.

The more you learn, the more fascinating the subject becomes.

Absolutely, and that’s how I found it right through my career.

In your earlier career, you concentrated on Neanderthals. Do you now accept the new evidence of Neanderthal-Homo sapiens interbreeding, which seems to establish that we are more than 2 percent Neanderthal?

This is one of the remarkable bits of news of the last couple of years. We’ve had the genomes of Neanderthals reconstructed, and yes, indeed, it shows that people outside of Africa have, on average, about 2.5 percent of an input of Neanderthal DNA in them. And, of course, it’s led to a rethinking of our relationship with them; clearly there was viable interbreeding.

We don’t know the circumstances. Maybe a parsimonious view is that there was a single interbreeding period when modern humans came out of Africa. They met some Neanderthals in the Middle East. There was some interbreeding, under circumstances we don’t know yet, and that input of Neanderthal DNA was then transferred as those populations spread to Europe and to China, down to New Guinea, into the Americas; they took that bit of Neanderthal with them.

Archaeologists have found evidence that Neanderthals and Homo sapiens occupied the same caves in Israel. Could this have been an interbreeding contact?

Western Asia becomes a critical area for this possibility of interbreeding. It could have been 25 Neanderthals mixing with 1,000 modern humans. It doesn’t have to be a lot of Neanderthals, but clearly there might have been interbreeding somewhere like Israel or Lebanon or Syria — all possible places where we know Neanderthals lived, and at times modern humans also lived.

There’s also a view that the interbreeding was more widespread, but that either cultural or physiological factors limited the successful births. For example, we know that the pelvic shape of Neanderthal females is different from the pelvic shape of modern human females. If a modern human female was giving birth to a hybrid baby, part Neanderthal, could there have been obstetric problems? We don’t know the circumstances of these encounters: if it was a peaceful mixing and merging of these people, or if the circumstances were violent.

Just who were the Denisovans?

It’s an extraordinary discovery. Two or three years ago I vaguely knew there was an archaeological site in Siberia called Denisova Cave. And then a few teeth, a finger bone have produced a really high-quality genome now that’s posted on the Web site of the Max Planck Institute for Evolutionary Biology in Leipzig, Germany. The preservation of the DNA is exceptional, and well beyond anything we have from Neanderthals. It seems these Denisovans were related to the Neanderthals, an early branch off the Neanderthal line.

We know a lot about the Denisovans genetically, but physically we know very little about them. These fossils are so fragmentary. The even more remarkable thing is they are only known from one site in Siberia, and their DNA turns up in people only in really one region today — not in Siberia, or Asia, but down in Australia and New Guinea. That’s extraordinary.

This is difficult to explain, because we thought that the ancestors of the Australian Aborigines and New Guineans must have got to their regions through southern Asia. Somewhere in Southeast Asia is the most likely place they would have had interbreeding with the Denisovans. That also implies the Denisovans were not just in Siberia; they must have been a widespread group.

This raises one more question: Could we ever clone these extinct people?

Science is moving on so fast. The first bit of Neanderthal mitochondrial DNA was recovered in 1997. No one then could have believed that 10 years later we might have most of the genome. And a few years after that, we’d have whole Denisovan and Neanderthal genomes available. So no one would have thought cloning was a possibility. Now, at least theoretically, if someone had enough money, and I’d say stupidity, to do it, you could cut and paste those Denisovan mutations into a modern human genome, and then implant that into an egg and then grow a Denisovan.

I think it would be completely unethical to do anything like that, but unfortunately someone with enough money, and vanity and arrogance, might attempt it one day. These creatures lived in the past in their own environments, in their own social groups. Bringing isolated individuals back, for our own curiosity or arrogant purposes, would be completely wrong.

In the introduction to your book, you list the kinds of questions you’re always getting from people. One of them will be the closing question: What is the future of human evolution?

That’s a tough one to answer. There’s a lot of data, not my research, but mainly geneticists have been working on this, and they’ve showed just how many genetic changes there have been in the last few thousand years in the human genome. And this is because we’ve undergone great changes with urbanization, with agriculture, very big changes in lifestyles. And this has influenced our genetic makeup as much as living in the Paleolithic had done. We’ve seen, if anything, an acceleration of genetic changes in humans due to these lifestyle changes. So, I think human evolution has been going on quite rapidly recently, and it’s going to carry on.

Not everyone agrees. My colleague in London, Steve Jones, has argued essentially that evolution has stopped in humans because we are in control of it. We have medical care. Nearly everyone reaches reproductive age. Everyone has enough food and water. So natural selection has been nullified in humans. I disagree with him because, of course, there are still a lot of people in the world who don’t have the best medical care, who don’t have enough food and water. Think of the impact of AIDS in Africa.

So selection is still operating on many human populations just as much as it ever has done, really. Also, all of us probably have 50 mutations in our DNA compared with our parents. So that’s going on every generation as well. We are still evolving. We will continue to evolve.

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